1. Field of the Invention
The present disclosure relates to a sheet stacking apparatus having the function of aligning sheets stacked on a stacking tray.
2. Description of the Related Art
Conventionally, there has been provided a system in which a sheet post-processing apparatus (finisher) is connected to the downstream side with respect to the sheet conveyance direction of an image forming apparatus, such as a copying machine, making it possible to perform post-processing, such as staple processing or punching processing.
In the finisher, sheets received from the image forming apparatus are successively stacked on an intermediate tray (hereinafter referred to as the processing tray) provided on the upstream side of the stacking tray. There has been discussed a finisher which performs post-processing, such as stapling and saddle binding, on the sheets stacked on the processing tray after the stacking of all the sheets constituting a booklet has been completed. The sheet bundle on which post-processing has been completed on the processing tray is discharged from the processing tray onto the stacking tray.
Japanese Patent Application Laid-Open No. 2001-240295 discusses a finisher in which sheets received from an image forming apparatus are discharged onto a stacking tray without being passed by way of the above-described processing tray, and then performs alignment processing in a width direction that is orthogonal to the discharge direction by alignment members provided on the stacking tray.
In an apparatus in which sheet alignment is performed on a stacking tray as in the case of the apparatus discussed in Japanese Patent Application Laid-Open No. 2001-240295, an alignment operation by an alignment member is performed each time a sheet is discharged. However, in the case of a thin paper sheet (e.g., a sheet whose grammage is less than 64 g), the following phenomenon may occur when the sheet is discharged to the exterior via a discharge outlet of the finisher due to the lack of strength (stiffness) in the conveyance direction and to the lightness of the sheet. More specifically, as compared with the case of a sheet of larger grammage, the thin paper may cause deviation in alignment timing due to the slowness in the falling of the sheet and leaning of the sheet on the discharge outlet, resulting in deterioration in stacking property.
The alignment property can be improved by delaying the alignment timing in synchronization with the falling of the thin sheet from the discharge outlet. However, when the alignment timing is delayed, it will be necessary to enlarge the sheet interval between the sheet being aligned and the next sheet to be received from the image forming apparatus, resulting in deterioration in productivity.